I would advise not using a back garden because it is really easy to miss and it doesn't take much to distract an archer, especially if you have not had practice at a range.
To answer your question, a standard archery range is between 12 and 16m. When I last taught it (this summer) I had kids shooting at targets between 8 and 20, and even the best archers amongst staff and students would not hit it 3 times out of 5 (Its classed as a fluke if any less in my eyes)
When I was learning to shoot I went out to a farmers field near me and asked the owner if I could shoot at his hay bails. Always shooting away from animals and any roads I learnt quickly enough and knew I wanted to stick with it.
If you are in the city, colleges(university's) tend to have archery clubs. You will have to contact them and maybe even get insurance for it, but that is the way I train at the moment.
Whatever you do, just be safe about it. You only need 1 arrow to go wrong.